Dual particle electrophoretic display and method of manufacturing same

ABSTRACT

Single particle and dual-particle electrophoretic display devices are disclosed. The display comprises a back substrate and including a substantially hollow container that includes a transparent substrate forming a cavity therebetween. The transparent substrate including one or more cathode electrodes forming a plurality of electronically and selectively addressable pixels; one or more side walls extending from the transparent substrate, the side walls defining corresponding pixels, and a suspension fluid including a plurality of pigment particles in fluid communication with each of the cells by a gap formed between the top of the side walls and the back substrate. In addition, the displays include a thin-film transistor (TFT) active matrix substrate to selectively drive one or more cathode electrodes. In addition, methods for manufacturing of the displays are disclosed.

RELATED APPLICATIONS

This application is a continuation of and claims priority to “SingleParticle Electrophoretic Display and Method of Manufacturing Same” filedon Feb. 15, 2011, and assigned application Ser. No. 12/931,983, now U.S.Pat. No. 8,436,807, and this application relates to “Methods ofManufacturing Electrophoretic Displays” filed on even date herewith, andassigned application Ser. No. 12/932,089, the entire contents of each ofwhich are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to display devices, and, moreparticularly, to electrophoretic display devices.

BACKGROUND INFORMATION

The electrophoretic effect operates on the principle that whenelectrophoretic particles are electrically charged to a particularpolarity, the charged electrophoretic particles will migrate from asurface being charged to the same polarity as the charged particlestoward a surface charged to a polarity opposite to that of the chargedparticles. For example, particles which are positively charged willmigrate from a positively charged surface toward a negatively chargedsurface.

Display devices that utilize the electrophoretic effect are known aselectrophoretic image displays (EPIDs). For example, U.S. Pat. No.7,289,101, titled “Multi-Color Electrophoretic Image Display”, whichissued on Oct. 30, 2007, and is assigned to CopyTele, Inc., isillustrative of an EPID. The EPID includes a plurality of cells, eachcontaining electrophoretic particles, capable of displaying differentcolors when the particles in the cell move from a first position (e.g.,rest) to a second position (e.g., display) in the cell. An electrode iscoupled to each of the cells and is operative to move the particles fromthe first position to the second display position (or from the secondposition to the first position) when properly biased. In this EPID, theelectrophoretic particle-containing cells are in fluid communicationwith one-another. In other types of EPIDs the particles are containedwithin sealed cells. The cells, whether sealed or partially open, (alsoreferred to as a pixel) may be in a round, a square, a rectangularand/or honeycomb shape or other similar shapes that allow for maximizingthe number of cells per unit area (e.g., hexagon, octagon).

The electrophoretic particles used in EPIDs may comprise light colored(light reflective) and/or dark colored (light absorbing) dielectricparticles that are suspended in an optically contrasting clear fluidmedium (suspension liquid). For example, U.S. Pat. No. 6,113,810,titled, “Methods Of Preparing Electrophoretic Dispersions Containing TwoTypes of Particles With Different Colors and Opposite Charges,”, andassigned to CopyTele, Inc., describes a dielectric dispersion for use ina electrophoretic display that includes a dielectric fluid, a firstplurality of particles of a first color having a surface charge of aselected polarity dispersed within the dielectric fluid and a secondplurality of particles of a second color having a surface charge ofopposite polarity to that of the first plurality and a steric repulsionthereto preventing coagulation of the first and second pluralities. Asunderstood by those skilled in the art, the electrophoretic particlesdescribed herein may have optical properties that extend from totallylight reflective (i.e., white) to totally non-reflective, lightabsorbing or opaque (i.e., black). Thus, reference to light coloredparticles refers to particles that have a greater light reflectiveproperty than a light absorbing property and dark colored particlesrefers to particles that have a greater light absorbing property thanlight reflecting property.

In accordance with the electrophoretic effect described above, theelectrophoretic particles in the suspension liquids (fluid medium)selectively migrate to, and impinge upon, a transparent screenelectrode, thereby displacing the fluid medium from the screen andcreating the desired image.

EPIDs have many advantages over other types of flat panel displays. Forexample, EPIDs are composed of materials that are relativelyinexpensive, and thus, the EPIDs are less costly to manufacture. Anotheradvantage is that the image formed on the screen remains even when poweris removed. When the electrophoretic particles or dye particles move toform an image, the image will not erase and remains on the display evenafter power is removed. Thus, the images created by EPIDs do not have tobe refreshed as is necessary for images produced by Liquid CrystalDisplays (LCDs) and other types of displays.

However, because an image is created by the movement of theelectrophoretic particles within the fluid when the display is properlybiased, the response time to view an image is dependent upon the time ittakes the particles to move from a rest position to a display position.

Hence, there remains a need for an EPID with a faster response time thatprovides gray scale imaging that can be manufactured for low cost, andmethods for manufacturing same.

SUMMARY OF THE INVENTION

A TFT based electrophoretic display is disclosed. The display comprisesa first and second substrates that include a plurality of cellstherebetween. Each of the cells containing electrophoretic particlescapable of displaying at least one different color when the particles inthe cell move from a first position to a second position within the celland an electrode coupled to each of the cells, which when biased causesthe particles to move from the first position to second display position(or from the second position to the first position). In one aspect ofthe invention, the cells are partially contained between the first andsecond substrates to allow for fluid communication of an electrophoreticfluid substantially particle free among the cells.

In another aspect of the invention, a method of filling an EPID withelectrophoretic fluid is disclosed. In this aspect of the invention,pigment particles are deposited on a first substrate and joined to asecond substrate to form a cavity between the first and secondsubstrates. A vacuum is created between a first and second substrate,which includes a plurality of cells that are formed with side wallsextending from one of the first and second substrates partially towardthe other substrate. A voltage is applied to the first substrate as asubstantially particle free fluid enters and is distributed throughoutthe cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrativeembodiments now to be described in detail in connection with theaccompanying drawings, wherein:

FIGS. 1A and 1B illustrate a cross-sectional view of an exemplary EPIDsaccording to an aspect of the invention;

FIG. 2 illustrates a composite view of a transparent TFT active matrixsubstrate according to an aspect of the invention;

FIG. 3 illustrates a cross-sectional view of an EPID according to anaspect of the invention;

FIG. 4 illustrates a cross-sectional view of an EPID according to anaspect of the invention;

FIGS. 5A and 5B illustrate cross-sectional views of a dual-particle EPIDaccording to an aspect of the invention;

FIGS. 6A and 6B illustrate cross-sectional views of another aspect of adual-particle EPID according to an aspect of the invention;

FIG. 7 illustrates a cross-sectional view of an EPID including cellparticle containment wall according to an aspect of the invention;

FIG. 8 illustrates a flowchart of a first method of manufacturing anEPID according to an aspect of the invention; and

FIG. 9 illustrates a second method of manufacturing an EPID according toan aspect of the invention.

It is to be understood that the drawings are solely for purposes ofillustrating the various aspects of the invention and are not intendedto define the limits or scope of the invention. Throughout the drawingsand the specification, like reference numerals are used to indicatecommon features of the described devices and methods.

DETAILED DESCRIPTION OF THE INVENTION

The terms “a” or “an” as used herein are to describe elements andcomponents of the invention. This is done merely for convenience and togive a general sense of the invention. The description herein should beread to include one or at least one and the singular also includes theplural unless indicated to the contrary.

The term “comprises”, “comprising”, “includes”, “including”, “as”,“having”, or any other variation thereof, are intended to covernon-exclusive inclusions. For example, a process, method, article orapparatus that comprises a list of elements is not necessarily limitedto only those elements but may include other elements not expresslylisted or inherent to such process, method, article, or apparatus. Inaddition, unless expressly stated to the contrary, the term “or” refersto an inclusive “or” and not to an exclusive “or”. For example, acondition A or B is satisfied by any one of the following: A is true (orpresent) and B is false (or not present); A is false (or not present)and B is true (or present); and both A and B are true (or present).

FIG. 1A illustrates a cross-sectional view of an electrophoretic imagedisplay (EPID) 100, according to an aspect of the invention. The EPID100 includes a transparent substrate layer including a thin filmtransistor (TFT) active matrix substrate 10 constructed from a generallyplanar sheet of transparent material, for example, plastic or a glass,one or more transparent cathode electrodes 16 deposited on thetransparent substrate, one or more anode electrodes 18 extendingsubstantially perpendicular from the transparent substrate, a pluralityof electrophoretic pigment particles 22, a substantially clear, andsubstantially particle-free electrophoretic suspension fluid 36, and alayer 28 disposed on a back substrate 38 constructed from a generallyplanar sheet of plastic or glass, the back substrate 38 being oppositelydisposed from the transparent substrate layer 10. In this aspect of theinvention, the electrophoretic pigment particles 22 are illustrated anddescribed as being dark or black and the layer 28 as reflective (e.g.,white). However, it should be understood that white or light coloredparticles and a dark or light absorbing layer 28 may also be suitablefor use according to an aspect of the invention.

Substrates 10 and 38 are sealingly assembled together with spacers 52 toform a liquid and gas-sealed multi-cell enclosure 54 located between theTFT active matrix substrate 10, the back substrate 38 and electrodelayer 28, and the spacers 52. The enclosure 54 includes a spacedesignated as “S” located between the TFT active matrix substrate 10 andthe electrode layer 28 disposed on the inner surface 50 of the backsubstrate 38, into which an electrophoretic, substantially clearsuspension fluid 36 is deposited. The height of the space S between thesubstrate 10 and the electrode 28 is greater than the height of thewalls of the anode electrodes 18, thus permitting fluid communicationamong the cells 60 (FIGS. 1A and 1B). That is, anode electrodes 18,which form the walls of a corresponding cell 60, extend from substrate10 partially toward the oppositely disposed back substrate 38 and layer28, but do not meet or touch the back substrate 38. Thus, a gap oropening 90 is formed between the inner surface of layer 28 and the topof the anodes 18 to allow fluid 36 to be freely dispersed among thecells 60 and within the enclosure 54. The advantages of the gap formedbetween the top surface of the anodes 18 and the inner surface of layer28 in the manufacturing of the EPID are described herein. As fluid 36 issubstantially particle free and not viscous, only a small gap isnecessary to allow fluid 36 to be distributed throughout the cavity soas to fill each of the cells with substantially the same amount offluid.

The TFT active matrix substrate 10 and the cathode electrodes 16 aretransparent to allow light to pass therethrough. According to an aspectof the invention, the anode electrodes 18 are in the form of walls thatextend substantially perpendicular from the inner surface 14 of the TFTactive matrix substrate 10. Cathode electrodes 16 are deposited on theinner surface 14 of the TFT active matrix substrate 10 between, and areelectrically isolated from, the anode electrodes 18.

Each cathode electrode 16 is fabricated on an inner surface 14 of theTFT active matrix substrate 10 by progressively depositing onto theinner surface 14 an electrically conductive material. For example,indium-tin-oxide (ITO) is a suitable transparent material that may beused. Other suitable materials with similar transparent and conductiveproperties may also be utilized.

The thickness of the cathode is preferable in a range less than 1.5microns. An insulating material layer 20 composed of a material such as,SiO₂, or other equivalent insulating material, including but not limitedto SiO or SiN_(x) may be applied to the cathode electrodes 16 to protectand isolate the electrodes 16 on the TFT substrate 10 from being inphysical contact with the fluid 36. The insulating material layer 20preferably has a thickness in the range of 100 Å (angstroms) to 2000 Å(angstroms). The electrically conductive cathode 16, and the transparentinsulating material 20 may be deposited using conventional semiconductordeposition techniques. An additional isolation layer may also be appliedto the anode 18 to protect and isolate the anode 18. In preferredembodiments of the invention the particle sizes may be in the range often (10) nanometers to five (5) microns.

Cathodes 16 are electrically connected to corresponding TFTs on the TFTsubstrate 10 through well-known connection methods, which need not bediscussed in the detail (see for example, U.S. Pat. No. 7,289,101).Cathodes 16 are TFT-controlled conductors. When a selected voltage isapplied, electrophoretic particles in a corresponding cell move from thecathode 16 to a corresponding anode 18, or from the anode 18 back tocathode 16. Anodes 18 are maintained at a relatively stable,non-varying, voltage level. In one aspect, the voltage on anode 18 maybe set at a constant voltage between a fully “on” cathode 16 outputvoltage and a fully “off” cathode 16 voltage. In this manner, theelectrophoretic particles may be moved in one direction when the cathode16 is fully “on” and in the other direction when the cathode 16 is fully“off.”

The anodes 18 in the form of walls 18 may be composed of a conductivematerial, or from an insulating material with a thin layer of conductivematerial disposed thereon. It should be understood that the walls may beformed from any suitable material, provided a layer of suitableconducting material is disposed thereon. Although not shown, it would berecognized at an electrical connection may be made to the anode 18though well-known means. For example, the substrate may include aconductive layer, which is not shown in the figures, that is insulatedfrom the TFT and cathode layers that provide electrical contact to theanodes 18. The conductive layer may include vias, electrically isolatedfrom, and extending through the TFT and cathode layers to provide ameans for providing a voltage to the anode 18. In another aspect theanodes 18 may be in direct contact with a conductive layer, which isdeposited on another insulating layer deposited on the TFT/cathodelayer.

Referring to FIGS. 1A and 2, the anode electrodes 18, in the form ofwalls, are shown surrounding and interposed between each transparentcathode electrode 16 for form a cell 60. One or more cells 60 are formedby the anode electrodes 18 in conjunction with the cathode electrodes16, and resemble an egg-crate structure. From the viewing side 12 of thedevice, each cell 60 defines at least one pixel, and the correspondingcathodes define the pixel apertures 68 (FIG. 2). In one aspect of theinvention, a constant voltage may be applied to the cathode 16 for aselected time (i.e., a display frame time period) to set the particlesin one position or another position. In another aspect of the invention,a voltage to a corresponding cathode may be varied in accordance with aselected time modulation technique or a voltage modulation technique.For example, by placing a voltage on a cathode for selected periods oftime over a display frame period, a select number of the particles willmigrate from the cathode to the corresponding anode. As the number oftime periods increases, while maintaining a constant voltage, anincreased number of particles migrate to the anode. Similarly, byvarying the voltage over the frame time or selected period of the frametime, by a voltage modulation technique, only some of the particlesmigrate to the corresponding anode. Hence, a gray scale display may beobtained, i.e., not all pigment particles remain on the cathode 16 oranode 18 with the appropriate application of a fixed voltage over theselected time or a varied voltage over the selected time. For example,U.S. Pat. No. 4,833,464, titled “Electrophoretic Information Display(EPID) Apparatus Employing Grey Scale Capability,” which is assigned tothe assignee herein, discloses a time based method for providing grayscale displays. Although, gray-scaling using time-modulation orvoltage-modulation is typically associated with black-white EPIDs, itwould be recognized that such modulation methods may be appropriatelyapplied to each element of colored pixel (e.g., R, G, B) of a colordisplay to create different levels of color from each element.

By applying a voltage for a selected number of short time intervals overa display frame period, a display is provided that results in theincomplete removal of pigment from an associated selected pixel. Hence,that pixel appears darker than surrounding pixels, but not as dark as afull application of the voltage over the display frame period. Theamount of particles moved and, hence, the darkness of each pixel is afunction of the time duration during which appropriate voltages areapplied to the rows and columns of the TFT array. In this manner, atiming generator can cause different pixels to have different degrees ofdarkness or grey scale values by varying the time during which thevoltage is applied to the display (time-modulation). It should beunderstood that a controller, including, for example, a computer,microprocessor and/or dedicated hardware (e.g., ASIC, FPGA) may be usedto selectively apply a voltage or current to selected ones of thecathodes for selected time periods. Similarly, the controller may applya varying voltage to the selected cathodes for a selected time (frametime) or sub-units of the frame time.

Referring to FIG. 2, the display includes a plurality of pixels arrangedin a matrix of rows 13 a and columns 13 b. Each pixel comprises a TFTtransistor which is connected to a cathode 16 (pixel electrode).Additional electrodes of the TFT transistor are connected to the row 13aand column 13b electrodes of the associated pixel. During the updatingof an image, an appropriate voltage (selection voltage) can be appliedto any of the rows. When a row is selected, each column delivers aspecific voltage to the pixel electrode located at the particular rowand column intersection. The TFT serves as a voltage storage device,preserving the voltage at the pixel electrode, while the other rows inthe matrix are being updated. The voltage that is applied to the pixelelectrode (cathode) during the updating process (scanning process)relative to the anode electrode controls the back and forth movement ofthe charged electrophoretic particles between the anode electrode andthe pixel electrode (cathode) for the respective pixel in the row andcolumn matrix. It should be understood that a voltage source or currentsource may also be used to apply an appropriate voltage or current tothe anode and TFT elements (or the cathode 16 directly) to achieve apotential difference between the anode and cathodes. For example, thevoltage source may be a direct current source that generates a single ormultiple voltages, or an alternating current source that is rectified togenerate one or more direct current sources. The current source may be adirect current or alternating current source that provides appropriatevoltage to the corresponding electrodes.

In addition, a controller (shown as a row controller and a columncontroller) may be incorporated to selectively apply a voltage (orcurrent), from the illustrated voltage source, to selected TFTs incorresponding rows and columns of the TFT array. A suitable controllermay be computer or microprocessor including code which directs theoutput of the voltage source to one or more selected rows and columns.Alternatively, the controller may be dedicated hardware (ASIC, FPGA)that directs the output of the voltage source to one or more selectedrows and columns.

Referring back to FIG. 1A, when charged dark or black electrophoreticparticles 22 are attracted to one or more anode electrodes 18, light isallowed to travel through the substantially clear, substantiallyparticle free, electrophoretic suspension fluid 36 and is reflected offthe reflective white electrode layer 28 on the back substrate 38 makinga pixel appear white when viewed through viewing side 12 (FIG. 1) orpixel aperture 68 (FIG. 2). When the charged dark electrophoreticparticles 22 are attracted to one or more transparent cathode electrodes16, light is blocked, making the pixel appear black when viewed throughpixel aperture 68. By moving back and forth from the walls formed by theanode electrodes 18, to and from the cathode electrodes 16, theelectrophoretic particles 22 essentially act as a light shutter.

Advantageously, the one or more cells 60 formed by the walls of theanode electrodes 18, in conjunction with associated cathode electrodes16, tend to isolate the electrophoretic pigment particles 22 from oneanother, thereby improving the electrical, colloidal, operational, andlife-time stability of the EPID 100. Moreover, the cells 60 can beeasily dimensioned to provide hundreds of pixels per inch, therebyenabling one to obtain extremely fine resolution, and to create highresolution display capabilities.

Since the anodes 18 in the form of walls are viewable through theviewing surface 12, the thickness of the walls should be kept to aminimum width to provide a maximum amount of usable viewing surface. Inone aspect of the invention, the thickness of the anodes 18 may bedetermined as a function of the space between the TFT layer 10 and theinner surface of the back layer 38, including any additional layersdisposed thereon. In one aspect of the invention, the thickness of theanode 18 may be selected to be proportional to the distance (space)between the substrates that form the cavity. For example, thethickness/space ratio may be approximately 1:10, or less to effectivelyreduce areas that would not otherwise be usable on the display.

Referring to FIG. 1B, there is shown another exemplary EPID 100′ inaccordance with an aspect of the invention. In this illustrated example,one or more walls 47 extending substantially perpendicular from theinner surface of layer 28 are disposed substantially opposite from theanodes 18 in the form of walls extending from the transparent substrate10. In this illustrated aspect of the invention, which may be referredto as a split-wall configuration, walls 47 provide for furthercontainment of the particles 22 in a corresponding cell. As illustrated,a gap 90′ exists between the top surface of the anodes 18 and the loweredge of walls 47 to allow for the substantially uniform distribution offluid 36 throughout the display.

According to an aspect of the invention, the layer 28 (FIGS. 1A and 1B)disposed on the inner surface 50 of the back substrate 38 may be formedof a conductive material. In this aspect, application of a voltage tolayer 28 provides for initializing and/or resetting the display or thedeposition of particles during the manufacturing process. Application ofa voltage to an electrically conductive layer 28 further provides forthe initial distribution of the electrophoretic dark or black pigmentparticles 22. In one aspect of the invention, the walls 47 may be formedof a material with conductive properties similar to that of layer 28, ormay formed of a different material with different conductive (ornon-conductive) properties. For example, walls 47 may be anon-conductive material when layer 28 is a conductive material.

The particles described herein are organic or inorganic particlessuitable for use in electrophoretic displays. For example, the darkelectrophoretic pigment particles 22 may include, but are not limitedto: carbon black, carbon nanotubes, iron oxide black, lamp black,Zn—Fe—Cr brown Spinel, Magnesium Ferrite, green Spinel, chromium oxideGreen, Indanthrone Blue, Ultramarine Blue Dioxazine Violet, QuinacridoneViolet, Anthraquinoid Red, and Perylene Red. Light electrophoreticpigment particles 24 suitable for use in the EPIDs may include, but arenot limited to: titanium dioxide, zinc oxide, silica, zinc sulfide,calcium silicate, alumina hydrate, Diarylide Yellow, Arylide Yellow,Diarylide Orange, and Perinone Orange.

FIG. 3 illustrates an EPID 120, according to another aspect of theinvention. In this aspect, a black electrode layer 32 is disposed on aback substrate 38, and the electrophoretic particles are reflectivewhite particles 24. When the charged white reflective electrophoreticparticles 24 are attracted to a side wall of an anode electrode 18 froman associated cathode 16, the pixel appears black when viewed throughviewing surface 12 (the black electrode 32 is being viewed through thepixel aperture. Alternatively, when the charged reflective whiteparticles 24 are attracted to the transparent cathode 16, the light isreflected off the surface of the white particles, making the pixelappear white when viewed through viewing surface 12.

It should be understood that the polarity of the white particles 24 isdifferent than the polarity of the black particles 22 in FIGS. 1A and1B, and thus, the polarity of the voltage difference required for theEPID illustrated in FIG. 3 is opposite from the voltage differencedisclosed previously with regard to FIGS. 1A and 1B.

FIG. 4 illustrates an EPID 130 in accordance with another aspect of theinvention. In this aspect, an opaque black electrode layer 32 isdisposed on a back substrate 38. According to this aspect of theinvention, the reflective electrophoretic particles 26 a-c are coloredparticles. For example, the particles selected may be a particle thatreflects a red color, reflects a green color, and reflects a blue color,respectively. It should be understood that the colors described hereinare only for describing an aspect of the invention, and that particlesof other colors may suitably be used.

The reflective electrophoretic particles 26 a-c may be deposited on theTFT active matrix substrate 10 by a conventional electrophoreticdeposition process. As illustrated, the reflective electrophoreticparticles 26 a-c are isolated and separated from one another withinrespective cells 60. When any of the charged electrophoretic particles26a-c are attracted to a corresponding anode electrode 18 (wall) 18 froma corresponding cathode 16, the pixel appears black when viewed throughviewing surface 12 (or pixel aperture 68 in FIG. 2), as the blackelectrode 32 is being viewed. However, when any of the colored particles26 a-c are attracted to the transparent cathode 16, the light isreflected off the surface of the particles, making the pixel appear thecolor of the particle, e.g., red, green or blue, when viewed through therespective pixel aperture 68.

It should be understood that the walls 47 (FIG. 1B) may be incorporatedinto the EPID illustrated in FIGS. 3 and 4 to provide for furthercontainment of the particles 24 or 26 a-c, respectively.

FIG. 5A illustrates a cross-sectional view of an EPID 140 in accordancewith another aspect of the invention. In this aspect, which may bereferred to as a dual-particle EPID, a transparent cathode electrode 16is disposed on the TFT substrate 10 and an anode electrode layer 34 isdisposed on a back substrate 38. In addition, both dark (e.g. black)particles 22 and light (e.g., reflective white) particles 24 aredeposited on respective opposing surfaces. According to this aspect ofthe invention, for any given pixel, depending upon the respectivevoltages of the anode layer 34 and the transparent cathode 16, thereflective white particles 24 will be attracted to the cathode 16, andthe black particles 22 to the anode layer 34 on the back substrate 38,making the pixel appear white, or alternatively, the black particles 22will be attracted to the cathode 16, and the reflective white particles24 to the anode layer 34 on the back substrate 38, making the pixelappear black.

In this aspect, the one or more separation walls 62, in conjunction witha cathode 16, form a cell 60. Still referring to FIG. 5A, the separationwalls 62 are in the form of a frame or mesh surrounding each cathodeelectrode, as illustrated, or may surround a plurality of cathodeelectrodes, and are formed of a suitable material, for example, anon-conductive material, to contain particles within respective cells.The separation walls extend from the substrate 10 partially toward theelectrode layer 34 but do not meet or touch the electrode layer 34. Theopening or gap formed therebetween allows for fluid communication amongthe cells. It should be understood that walls 47 illustrated in FIG. 1Bmay also extend from the inner surface of electrode layer 34 to providefurther containment of the particles 22, 24 within a corresponding cell.This split-wall design also provides a gap through which fluid 36 may bedistributed among the cells. It should be understood that the separationwalls may also be made of a conductive material to allow for a voltageto be applied to attract particles, as previously described.

FIG. 5B illustrates a cross-sectional view of an EPID 150 in accordancewith another aspect of the invention. In this aspect, on the viewingside 13 an anode electrode layer 34 is disposed on a back substrate 38and a cathode layer 16 is deposited on a TFT layer 10, in which bothblack and reflective white particles, 22 and 24 are deposited onrespective opposing surfaces. According to this aspect of the invention,for any given pixel, depending upon the respective voltages of thetransparent anode layer 34 and cathode 16, the reflective whiteparticles 24 will be attracted to the cathode 16, and the blackparticles 22 to the transparent anode layer 34 on the back substrate 38,making the pixel appear black. Alternatively, the black particles 22will be attracted to the cathode 16, and the reflective white particles24 to the transparent anode layer 34 on the back substrate 38, makingthe pixel appear white. In this aspect, the one or more separation walls62, in conjunction with a cathode 16, form a cell 60. The separationwalls are in the form of a frame or mesh surrounding each cathodeelectrode or a group of cathode electrodes, and are formed of a suitablenon-conductive material, and are less than the height of space S toallow for the fluid communication between the walls as describedpreviously.

FIGS. 6A and 6B illustrate a cross-sectional view of another aspect ofthe invention including a dual-particle electrophoretic display.

According to this aspect, particles having two different electricalcharges are included within each cell. The anode electrodes 18 arecharged to a known voltage as previously described. When the voltageapplied to the cathode 16, through the corresponding TFT circuit is lessthan the voltage on the anode 18, particles having a first electricalcharge are attracted to the anode 18 (e.g., particles 22), whileparticles (e.g., 24) having a second electrical charge are attracted tothe cathode 16 (FIG. 6A). When the voltage applied to the cathode 16 isgreater than the voltage applied to the anode 18, then the particleshaving the second electrical charge (e.g., 24) are attracted to theanode 18, while the particles (e.g., 22) having the first electricalcharge are attached to the cathode 16 (FIG. 6B). Thus, in the formercase, the pixel would appear to have the color of the particles havingthe second electrical charge, while in the latter case, the pixel wouldappear to have the color of the particles having the first electricalcharge. While the most dramatic effect occurs when the particles areselected to be black and white, wherein the pixel is either black orwhite, when viewed from either side, it should be understood thatalternative operations may be performed using electrically chargedparticles with any combination of colors, ranging between fullyreflective and fully absorptive.

FIG. 7 illustrates a cross-sectional view of an EPID 180 according toanother aspect of the invention. The EPID 180 includes a TFT activematrix substrate 10, with a front viewing side 12, transparent cathodeelectrodes 16, and a plurality of pixel separation walls 92 disposedbetween and surrounding each cathode 16 (or a group of cathodes 16). Thecathode electrodes 16 and separation walls 92 are disposed on the innersurface 14 of the substrate 10. In this aspect, an insulation layer 20is not used, as in FIG. 1A. It should be understood that insulationlayer 20 may be optionally included in the EPIDs illustrated anddescribed herein.

The EPID 180 includes a back substrate 38 with a dark anode electrode 34disposed on the inner surface thereof. Disposed on the inner surface ofthe dark anode electrode 34 is a plurality of separation walls 94. Theseparation walls 94 are contained within the projection of theseparation walls 92 onto the back electrode. In this aspect, whiteelectrophoretic particles 24 may be suspended in a clear electrophoreticsuspension fluid 36, or may be deposited, prior to sealing the TFTactive matrix substrate 10 and the back substrate 38, on the TFTsubstrate 10 or the back substrate 38 by conventional electrophoreticdeposition processes. The EPID may then be filled with a clear,substantially particle-free electrophoretic suspension fluid 36 aftersealing the TFT active matrix substrate 10 and the back substrate 38together.

The cathode electrodes 16 are separated from one another by theseparation walls 92 that extend in a substantially perpendiculardirection from the TFT active matrix substrate 10. Pixel separationwalls 92 are in the form of a mesh-like structure. The separation walls94 disposed on the dark anode electrode 34 surround and cross eachcathode electrode 16 within a respective pixel area. Between twoadjacent walls 94 are wells 96 that contain and hide the white particles24. When the reflective white particles 24 are attracted to the anode34, the particles 24 are contained within the wells 96 and therespective pixel appears dark (the color of the walls 94). When thereflective white particles 24 are attracted to the cathode 16, therespective pixel appears white. The separation walls 94 are advantageousas they provide wells 96 for containment of the particles within thepixel. In addition, it should be understood that walls 47 illustrated inFIG. 1B may also be incorporated into the EPID 180 to provide forfurther containment of the particles within the cell.

According to another aspect of the invention, a method for manufacturingthe EPIDs herein is described and illustrated The method includes, priorto the step of sealing the TFT active matrix substrate 10 and the backsubstrate 38 about the perimeters thereof and filling the sealedcontainer or cavity with a substantially clear, substantiallyparticle-free, electrophoretic suspension fluid 36, a plurality ofelectrophoretic particles 22 may be deposited on the TFT substrate 10 orthe back substrate 38 by conventional electrophoretic depositionprocesses. This is accomplished because the height of the anodeelectrode 18 is less than the height of the space S which forms a gaptherebetween to allow the clear electrophoretic suspension fluid 36 tobe added in an efficient manner. The gap provides for fluidcommunication among cells. In one aspect of the invention, the height ofthe space S may be approximately 10 um and the anode height may beapproximately 7 um. The differences in height provides a sufficient gapso that in instances where the particles are deposited prior to fillingwith the electrophoretic suspension fluid, the cavity or hollow EPID canbe easily filled with the electrophoretic suspension fluid with the aidof a partial vacuum. A voltage may be applied to the substrate(s) duringthe deposition of the particles using conventional electrophoreticdeposition techniques to maintain the particles in place. In addition,during the filling process, a voltage may be re-applied to maintain thedeposited particles in place as the fluid is being added to the cavitybetween the first and second substrates.

The manufacturing of EPIDs is disclosed in U.S. Pat. No. 5,279,511,titled “Method of Filling an Electrophoretic Display,” which issued Jan.18, 1994, and is assigned to CopyTele, Inc. In this patent, twoelectrode plates separated by spacers create a cavity in which an fluidcontain electrophoretic particles are dispersed. A voltage difference isapplied to the electrodes to create an electrophoretic effect to collectthe particles at one electrode or the other. The particular electrode towhich the particles collect is determined by the particle charge and thevoltage difference. Thereafter, the fluid is drained, the device is thendisassembled to enable the particles to dry in place and then the deviceis reassembled. A clear suspension fluid is then introduced into theassembled EPID. The process has been found to be both time consuming andcostly as the step of disassembling and drying are additional steps thatincrease the cost and the amount of time to fabricate the EPID.

In accordance with an aspect the invention, electrophoretic particlesare deposited on an electrode plate (substrate) by the application of avoltage to the plate and then the plate is combined with a secondsubstrate to create a cavity between the two plates. In this case, thevoltage applied to the plate may be maintained or may be removed duringthe assembly process.

The cavity formed between the two plates may be void of any structure ormay include structure with cells that are formed with side wallsextending substantially perpendicular from one of the electrodes so asto form a gap between the top of the side wall and the opposing plate(electrode) (FIG. 1A) or the cells may be constructed using a split gapside wall (FIG. 1B). After sealing the cavity formed by the first andsecond plates (substrates) about the perimeter, a voltage difference isthen maintained between the two plates to retain the deposited particlesin place. A partial vacuum (i.e., a desired level of vacuum) is thencreated within the cavity by drawing the enclosed air out usingconventional methods of evacuation. A clear suspension fluid is theninjected, or drawn, into the partially evacuated cavity. In one aspectthe fluid may be injected under pressure into the cavity. In anotheraspect the fluid may be drawn into the cavity by a combination of thecreated partial vacuum and capillary action.

With the gap created between the cell side walls and the opposing plate,the clear suspension fluid may be distributed through the cavity. Inaddition, the applied voltage difference during the filling processcauses the deposited particles to remain in place. Hence, a more uniformdistribution of particles and suspension fluid is achieved, as theparticle placement is not disturbed as the suspension fluid occupies theunfilled space within the cavity. In another aspect of the invention,the filling process may be performed without the application of avoltage. In this case, the particles may be held in place by Vanderwaalforces.

In another aspect of the invention, the air may be evacuated from thecavity while the suspension fluid is concurrently injected or drawn. Inthis case, the rate of injecting the fluid must be more accuratelycontrolled to avoid the fluid being drawn out by the evacuating process.

FIG. 8 illustrates a flow chart 800 of an exemplary manufacturingprocess in accordance with a first aspect of the invention. In thisaspect, a single particle EPID is fabricated (e.g., FIG. 1A, 1B). Atstep 810, an electrically conductive plate (or substrate) is placed on aholder. At step 820, in one aspect of the invention, a voltage isapplied to the plate as a fluid containing electrophoretic particlescontacts the plate. The voltage causes the particles to be deposited,and be retained, on the surface to the plate. It should be understoodthat the voltage used depends on the electrical charge of particles thatare being deposited on the plate or substrate.

At step 830, the plate having particles deposited thereon is assembledwith, but separated from, a second plate to form a cavity therebetween.The two plates may be separated by spacers to maintain a desireddistance between the two plates (i.e., substrates). The spacers may beformed of a non-conductive material. Alternatively, the spacers may beformed of a conductive material that is electrically isolated from theelectrically conductive elements on one or both of the plates.

At step 840, the cavity is sealed and the air in the sealed cavity iswithdrawn. At step 850, a voltage may then be applied to the substrateto maintain the deposited particles in place as a clear suspension fluidis injected, or drawn, into the evacuated cavity. At step 860, theapplied voltage may be removed.

FIG. 9 illustrates a flow chart 900 of an exemplary manufacturingprocess in accordance with another aspect of the invention. In thisaspect of the invention, a dual-particle EPID (e.g., FIGS. 5 a-5 b and 6a-6 b) is fabricated. At step 910, an appropriate voltage is applied toeach of electrically conductive plates that are to be used as thesubstrates of the EPID in a manner as described with regard to step 810.At step 920, electrically charged particles are attached tocorresponding ones of the electrically charged plates. Steps 930-960 aresimilar to the steps 830-860 and, need not be further described.

The advantages of the EPID presented herein are:

1. The incorporation of a TFT substrate for rapid scanning operation.This allows for high resolution gray scale images with a very fastresponse time.

2. The partial walls that separate the pixel cells allow the EPID to befilled very efficiently. This is because the electrophoretic particlescan be deposited on the surface of the substrates prior to filling.Therefore when filling the EPID only a substantially particle freeelectrophoretic fluid needs to be added. This can be done because allthe cells are in fluid communication with one another. As the particlefree electrophoretic fluid is not viscous, only a small gap isnecessary. The small gap between the opposite electrode results in loweroperating voltages (because of the relatively higher electric fields),and a short distance between the anode and cathode electrodes. Thisshort distance translates into a fast response time because of thehigher electric field and the shorter distance that the particlestravel.

3. Since the separation walls are not very high they need not be verywide. This allows for a very high contrast because very little of thepixel area is wasted on the cell separation walls.

4. The separation walls also help contain the particles in a respectivepixel cell. The particle containment may be further enhanced if thereare partial separation walls on both the anode and cathode substrates.

The instant application has referred to US patents that have issued andare assigned to the Assignee of the instant application to providebackground materials regarding the subject matter claimed as theinvention. The teachings of the aforementioned referred-to US patentsare incorporated by reference, as if stated in full, herein.

While there has been shown, described, and pointed out, fundamentalnovel features of the present invention as applied to the variousembodiments thereof, it should be understood that various omissions andsubstitutions and changes in the apparatus and methods described, in theform and details of the devices disclosed, and in their operation, maybe made by those skilled in the art without departing from the spirit ofthe present invention. For example, it is expressly intended that allcombinations of those elements which perform substantially the samefunction in substantially the same way to achieve the same results arewithin the scope of the invention. Substitutions of elements from onedescribed embodiment to another are also fully intended andcontemplated.

For example, while the invention has been shown and described withregard to the substantially perpendicular anodes 18 and/or separationwalls 92 extending from the transparent electrode, it should beunderstood that the anodes 18 or separation walls 93 may extend from theback substrate according to an aspect of the invention. In addition,while the invention has been shown and described with regard to avoltage being applied to the cathode to control the disposition ofcharged particles, it should also be within the knowledge of thoseskilled in the art to apply the TFT based control voltage to the anodesto control the initial particle disposition, prior to filling with asuspension fluid. In addition, while the invention has been describedwith regard to a reflective or an opaque back substrate, it should alsobe within the knowledge of those skilled in the art to form a EPIDhaving a transparent back substrate, and a reflective and/or opaquesubstrate having cathode elements thereon.

What is claimed is:
 1. A display comprising: a first substrate,including: a plurality of first walls extending substantiallyperpendicular from an inner surface of said first substrate, saidplurality of first walls having a top surface substantially parallel tosaid first substrate, said plurality of first walls forming a pluralityof cells; a plurality of electrically conductive elements, eachconductive element being associated with a corresponding one of saidplurality of cells, said electrically conductive element beingelectrically isolated from each of said plurality of walls; a secondsubstrate, separated from the first substrate by a known distance andforming a cavity between said first and said second substrate, aplurality of second walls extending from said second substrate, saidplurality of second walls having a lower edge substantially parallel tosaid second substrate, said plurality of second walls disposedsubstantially opposite from corresponding ones of said plurality offirst walls, said plurality of first walls having a height less thansaid cavity,—wherein a gap is formed—between the top surface of thefirst walls and the lower edge of a corresponding one of the secondwalls, and a thickness of the plurality of first walls is proportionalto the known distance between the first substrate and the secondsubstrate, said thickness of said first walls being sized to decreaseunusable display area; a substantially clear, substantially particlefree, suspension fluid contained within said cavity, said suspensionfluid being substantially uniformly distributed throughout said cavityby being in fluid communication with each of said cells; a firstplurality of electrically charged particles and a second plurality ofelectrically charged particles provided within each of said plurality ofcells, said first plurality of electrically charged particles having adifferent charge than said second plurality of electrically chargedparticles, said gap being sized to contain said first set and second setof electrically charged particles within corresponding cells; and meansfor selectively applying a voltage to each of said plurality ofelectrically conductive elements.
 2. The display of claim 1, furthercomprising; an insulating layer deposited on said electricallyconductive elements.
 3. The display of claim 1, further comprising: anelectrode layer deposited on an inner surface of said second substrate.4. The display of claim 3, further comprising: means for applying avoltage to said electrode layer.
 5. The display of claim 4, wherein saidmeans for applying a voltage is operable to apply a voltage to acorresponding one of said plurality of electrically conductive elementsgreater than a voltage applied to said electrode layer.
 6. The displayof claim 4, wherein said means for applying a voltage is operable toapply a voltage to a corresponding one of said plurality of electricallyconductive elements less than a voltage applied to said electrode layer.7. The display of claim 1, wherein the particles in said first set ofparticles are dark.
 8. The display of claim 1, wherein the particles insaid second set of particles are light.
 9. The display of claim 8,wherein said light particles are selected from the group consisting of:white and colored.
 10. The display of claim 9, wherein said coloredparticles are selected from the group consisting of: red, blue, andgreen.
 11. The display of claim 1, wherein said first substrate and saidplurality of electrically conductive elements are selected from thegroup of: transparent and opaque materials.
 12. The display of claim 1,further comprising: a TFT structure disposed on said inner surface ofsaid first substrate, said TFT structure being arranged in a pluralityof row and columns, wherein an intersection of one of said rows and oneof said columns corresponds to one of said plurality of electricallyconductive elements.
 13. The display of claim 1, further comprising aplurality of walls extending from said back substrate, said plurality ofwalls lying within a projection of said plurality of first walls on saidback substrate.
 14. The display of claim 1, wherein said cavity issealed about its perimeter.
 15. The display of claim 1, wherein theplurality of first electrically charged particles contained within eachof said cells has a size in the range of 10 nanometers to 5 microns. 16.The display of claim 1, further comprising means for applying a voltageto said first walls.
 17. An electrophoretic display device, comprising:a back substrate; a substrate, being selected from the group oftransparent and opaque materials, positioned substantially opposite fromsaid back substrate forming a substantially hollow containertherebetween, said substrate comprising; a plurality of cathodeelectrodes supported by said substrate forming a plurality ofelectronically and selectively addressable pixels; a plurality ofseparation walls extending from said substrate, said separation wallsinterposed between corresponding cathode electrodes and having a topsurface substantially parallel to said substrate; a plurality of secondwalls extending from said back substrate, said plurality of second wallsdisposed substantially opposite from corresponding ones of saidplurality of separation walls, said second walls having a lower edge,wherein a gap is formed between the lower edge of said plurality ofseconds walls and the top surface of corresponding ones of saidplurality of separation walls; and a thickness of the plurality ofseparation walls is proportional to a distance between the backsubstrate and the substrate, said thickness of said separation wallsbeing sized to decrease unusable display area; a plurality of pigmentparticles in each of said cells, the pigment particles in each of saidcells comprised of light particles and dark particles; a substantiallyclear, substantially particle free, suspension fluid disposed in saidcontainer, said suspension fluid being substantially uniformlydistributed through said container through said gap, said gap beingsized to contain said first set and said second set of electricallycharged particles within corresponding cells; and means for selectivelyapplying a voltage to each of said plurality of cathode electrodes. 18.The device according to claim 17, wherein the substrate comprises athin-film transistor active matrix substrate comprising one or morethin-film transistor driver circuits, each being electrically coupled toan associated pixel.
 19. The device according to claim 17, wherein saiddark pigment particles are black.
 20. The device according to claim 17,wherein said light pigment particles are selected from the groupconsisting of: white and colored.
 21. The device according to claim 20,wherein said colored particles are selected from the group consistingof: red, green, and blue.
 22. The display of claim 17, wherein saidseparation walls extending from said substrate are composed of amaterial selected from a group of: electrically conductive andelectrically non-conductive materials.
 23. The display of claim 22,wherein said separation walls are coated with an electrically conductivematerial.
 24. The device of claim 17, wherein the particles containedwithin each of said cells has a size in the range of 10 nanometers to 5microns.
 25. A display comprising: a first substrate, including: a TFTlayer including a plurality of Thin Film Transistors (TFT) arranged inan array of rows and columns; a plurality of first walls extendingsubstantially perpendicular from said first substrate, said plurality offirst walls having a top surface substantially parallel to said firstsubstrate, said plurality of first walls creating a plurality of cellscorresponding to said array of TFTs; an electrically conductive elementwithin each of said cells in electrical contact with a correspondingTFT, said first walls and said electrically conductive element beingelectrically isolated; an insulating layer deposited on said firstsubstrate; a second substrate, separated from the first substrate by aknown distance to form a cavity between the first and second substrate,a plurality of second walls extending from said second substrate, saidplurality of second walls disposed substantially opposite fromcorresponding ones of said plurality of first walls, said second wallshaving a lower edge substantially parallel to said second substrate,wherein a gap is formed in between said lower edge of said plurality ofseconds walls and the top surface of corresponding ones of saidplurality of first walls and a thickness of the plurality of first wallsis proportional to the known distance between the first substrate andthe second substrate, said thickness of said first walls is sized todecrease unusable display area caused by said first walls; a clear,substantially particle free, suspension fluid contained within saidcavity, said suspension fluid being substantially uniformly distributedthroughout said cavity by being in fluid communication with each of thecells through said gap, said gap being sized to contain said first setand second set of electrically charged particles within correspondingcells; a first set and a second set of electrically charged particlescontained within each of said cells, said first set of electricallycharged particles having a different electrical charge that the secondset; and means for applying a voltage to selected ones of the TFTs inthe TFT array to apply a voltage to corresponding ones of theelectrically conductive elements.
 26. The display of claim 25, furthercomprising: means for applying a voltage to said second substrate. 27.The display of claim 25, wherein the particles in said first set ofparticles are dark.
 28. The display of claim 25, wherein the particlesin said second set of particles are light.
 29. The display of claim 28,wherein said light particles are selected from the group consisting of:white and colored.
 30. The display of claim 29, wherein said coloredparticles are selected from the group consisting of: red, blue, andgreen.
 31. The display of claim 25, wherein said first substrate andelectrically conductive elements are transparent.
 32. The display ofclaim 25, wherein said second substrate is transparent.
 33. The displayof claim 25, wherein the particles contained within each of said cellshas a size in the range of 10 nanometers to 5 microns.
 34. The displayof claim 25, further comprising means for applying a voltage to saidfirst walls.